Communication devices such as User Equipments (UE) are also known as e.g. mobile terminals, wireless terminals and/or mobile stations. User equipments are enabled to communicate wirelessly in a wireless communications network, sometimes also referred to as a wireless communication system, a cellular communications network, a cellular radio system or a cellular network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
User equipments may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The user equipments in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another user equipment or a server.
The wireless communications network covers a geographical area which is divided into cell areas, wherein each cell area is served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB” or “B node” depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations. 3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE is controlled by the radio base station.
Heterogeneous Networks
A promising and practical approach to increase the throughput for UEs in a cellular communications network is by using heterogeneous networks (HetNets). The idea is to deploy smaller base stations where data requirements are the highest in order to provide a good service even in crowded geographical areas, where it is not possible to deploy more standard base stations. These smaller base stations are much cheaper and transmit with a much lower power than the standard base stations.
Types of Base Stations in HetNets
Macro nodes are the standard base stations, as deployed nowadays. Macro nodes transmit with the highest power and may therefore serve UEs at the furthest distance. Macro nodes typically use a transmit power of 40 W and are designed to cover larger areas like city districts. An area comprising all UEs served by a macro node is called a macrocell.
Pico nodes are smaller base stations than the standard base stations, and have a significantly lower transmit power. Pico nodes serve UEs in much smaller geographical areas, e.g., a mall or a metro station. Such a geographical area with a high UE density may be called a hotspot. Pico nodes typically use a transmit power of 1 W. An area comprising all UEs served by a pico node is called a picocell.
Femto nodes are the smallest base stations. They are typically used to cover a small office or a house.
In the following only the terms macro and pico nodes will be employed to describe high and low power nodes.
Cell Selection in HetNets
When a UE needs to join the wireless communications network, it first has to find which are the cells in its neighborhood and then select which one of them it will be associated to.
The role of the cell selection algorithm is to decide which cell to connect to, based on measurements performed on multiple cells within range for the UE.
The cell selection algorithm is typically a cell-based algorithm, which means that it affects all UEs in the cell, each time it is processed. Typically the cell selection occurs periodically, when it is assumed that the channel conditions have changed drastically, and also based on the network itself, i.e., when the UE enters or leaves the cell, i.e. when the UE is handed over to another cell.
There exist several methods to associate UEs to cells. For example, at the time a UE needs to be associated to a base station, it measures the received power from each base station within range of such measurements. This UE may then be associated to the base station from which the largest power was received. This may be referred to as the Reference Signal Received Power (RSRP) method.
Another method is the Cell Range Extension (CRE) method. The main idea of the CRE method is to virtually increase the range of the pico nodes by a fixed factor. The difference with respect to the RSRP method is that the UE is only associated to a macro node if the received power from the macro node is higher than the received power from the pico node plus the fixed extension factor. In other words, for cell selection the CRE value is an offset value between two cells, such as a pico cell and a macro cell.
The range extension enables to balance the distribution of the users between the macro and pico nodes. In this way the pico node is guaranteed to be serving a significant amount of UEs.
However, in heterogeneous networks cell selection is particularly complex due to the power imbalance between the high power nodes, e.g. macro nodes, and the low power nodes, such as pico or femto nodes. Pushing UEs toward the low power nodes results in a poor downlink Signal to Interference and Noise Ratio (SINR) for cell edge UEs. Pushing UEs towards the high power nodes results in an underutilization of the bandwidth resources and in a poorer uplink channel quality than could have been achieved for the UE in the low power node.